EW63/6Ghz Waveguide Sweep

I am trying to sweep a 6 Ghz transmission line system (EW63 and dish mounted on a tower) using the Spectrum analyzer on a fieldfox N9916A (using CAT) How should a good sweep look? I would expect a good system (about 200 feet of EW63 and a dish) would be around 25 DB return loss? The sweeps that I made were not flat across the sweep 5-8 ghz. I would have expected the display to be much flatter. The top of the reflected response (peaks) were below -20 db, but I had about 15 db of signal below that? Again, I would expect that to be reasonably flat. Could someone point me to what a good sweep should look like (sweep of waveguide and dish)?